如图:
设AE=x;
则 AC = AE + CE = x + 3/4x = 10.5
所以 x = 6;即AE= 6;EC = 10.5 - 6 = 4.5
因为中间有个正方形,所以DE = EF,
DE = AE*cos∠AED,EF = CE * sin∠ECF
所以 AE*cos∠AED = CE * sin∠ECF
6*cos∠AED = 4.5 * sin∠ECF,
又∠AED = ∠ECF
所以sin∠ECF / cos∠AED = tan∠ECF = 6/4.5 = 4/3
sin∠ECF = 4/根号(3*3+4*4)=4/5;
cos∠ECF = 3/5;
EF = EC * sin∠ECF = 4.5 * 3/5 = 2.7;
FC = EC * cos∠ECF = 4.5 * 4/5 = 3.6
三角形CEF 的面积 为 : 1/2 * EF * FC = 4.86;
AD = AE * sin∠ECF = 6 * 3/5 = 3.6;
DE = AE * cos∠ECF = 6 * 4/5 = 4.8
三角形ADE 的面积 为 :1/2 * AD * DE = 8.64
正方形除外部分的面积:4.86+8.64 = 13.5